Patent Document 1 discloses a solid-polymer-type fuel cell comprising a polymer electrolyte membrane with a plurality of fine convex portions on its surface. The fine convex portions increase a reaction area and improve an efficiency of a catalyst. Patent Document 1 discloses press-molding method and casting method to form the fine convex portions on the surface of the polymer electrolyte membrane.
In the casting method, an electrolyte solution containing a polymer electrolyte or its precursor as well as a solvent is cast on a mold with a plurality of fine concave portions on its surface. Subsequently, a solidified polymer electrolyte membrane is peeled off from the mold. The resultant polymer electrolyte membrane comprises a mirror-structure of the mold on its surface.
Patent Document 2 and Non-patent Document 1 disclose a method for forming a polymer electrolyte with a plurality of fine convex portions. According to these documents, an electrolyte solution containing a monomer of a polymer electrolyte membrane and a polymeric initiator is cast on a mold made from polycyanomethylacrylate with a plurality of fine concave portions. Subsequently, the monomer is polymerized by irradiating ultraviolet, and a solidified polymer electrolyte membrane is peeled off from the mold.
Patent Document 3 discloses a method for fabricating a polymer electrolyte membrane comprising a catalyst layer with a plurality of fine convex portions. According to Patent document 3, the polymer electrolyte membrane is fabricated by dropping a polymer electrolyte aqueous solution on an alumina-nano-hole array comprising a catalyst layer and coating a polymer electrolyte membrane thereon.
Patent Document 4 discloses a method for forming a plurality of fine convex portions on the surface of polyimide fluoride resin using a casting method, as a method for forming a membrane of a resin other than polymer electrolyte membrane for fuel cell. According to Patent Document 4, hydrophilic treatment is conducted on the surface of a silicon mold with a plurality of fine concave portions, and a solution containing resin monomer is cast on the silicon mold. Subsequently, the resin monomer is polymerized with heat, and immersed in warm water. Finally, the resultant polyimide fluoride resin is peeled off from the mold. The hydrophilic treatment weakens the adherence between the solidified resin and the silicon mold, and promotes the peeling.